In an optical amplifier including a rare-earth doped fiber and an excitation light source, a ratio between light levels detected in a light input monitoring unit and an output monitoring unit that include optical branching couplers and photodiodes, namely, an optical gain, is maintained at a given level owing to automatic gain control.
An amplifier alarm circuit detects an increase in spontaneous emission light output from the optical amplifier. A signal component in a predetermined wavelength band passes through a band pass filter to remove the wavelength component of an optical signal and the power of the signal component is compared with a threshold value. When power exceeding the threshold value has been detected, it is determined that spontaneous emission light has increased, and an output abnormality alarm is output.
In an optical transmission system, a case where the optical power of light having been received is smaller than a predetermined threshold value is determined as an abnormality. Each of a signal light wavelength component and a predetermined noise light component is extracted from transmitted light, the ratio of optical powers of the signal light wavelength component and the predetermined noise light component is compared with a predetermined value, and the deterioration of transmission quality is detected.
A related technique is disclosed in Japanese Laid-open Patent Publication No. 2002-368698, Japanese Laid-open Patent Publication No. 2004-6887, Japanese Laid-open Patent Publication No. 10-209967, or Japanese Laid-open Patent Publication No. 11-186962.